These studies are concerned with various assessments of the acute and long term effects of radiation and chemotherapy on hematopoietic stem cells. Acute survival characteristics and changes in differentiation have been studied. Stimulation for proliferation (recruitment) has also been determined. Most recent studies are concerned with whether hematopoietic stem cells have limited proliferative capacities and if so, how these are affected by a variety of cytotoxic agents. BIBLIOGRAPHIC REFERENCES: Bloomer, W.D. and Hellman, S.: Normal Tissue Responses to Radiation Therapy, N.E.J.M. 293: 80-83, 1975.